With the advance of information technology, information-communication technology, and multimedia technology in recent years, there has generally been an increasing need for an optical information recording medium having a higher density and a higher capacity. An upper limit of a recording density of the optical information recording medium is mainly determined by a diameter of a spot of a light beam for reproducing information.
The diameter of the spot of the light beam is approximately expressed by λ/NA, where λ is a light source wavelength of reproducing light and NA is a numerical aperture of an objective lens for determining the diameter of the spot of the light beam. It is possible to improve the recording density of the optical information recording medium by reducing the diameter of the spot of the light beam.
However, it is considered that reduction in the light source wavelength λ of the reproducing light is limited to a wavelength of ultraviolet light, due to (i) absorption by an optical element and (ii) restriction given by a sensitivity property of a detector.
Further, an improvement of the NA is substantially limited by an allowance of inclination of the optical information recording medium. Therefore, there is a limit on an improvement in the recording density of the optical information recording medium by reduction of the diameter of the spot of the light beam.
As a technique for overcoming these limits, a super-resolution reproducing technique is generally known. The super-resolution reproducing technique is a technique for reproducing a signal having a mark length shorter than an optical resolution limit (i.e., a mark length of approximately λ/(4NA), which is a length one-fourth of a diameter of a light spot) of a reproducing device. This technique allows an even shorter mark length to be recorded on an optical information recording medium. This improves a recording density of an optical information recording medium.
Patent Document 1 discloses a super-resolution optical information recording medium, which is called Super-ROM. The super-resolution optical information recording medium uses a high-resolution technique using a prepit, which makes a protrusion/depression pattern and contributes to a reproduction of information. At the present time, detail of a mechanism of how a reproduction is carried out by the super-resolution optical information recording medium disclosed in Patent Document 1 has still not been elucidated.
However, in a case where a reflecting film of a reproduction-dedicated disc is made of material such as Mo, W, Si, and Ge, instead of a conventionally-used reflecting film made of Al or Au, it is possible to reproduce a signal having a short recording pit length which is an optical resolution limit of a conventional optical system.
Patent Document 2 discloses a super-resolution reproducing method available to a rewritable magneto-optical recording medium. However, this super-resolution reproducing method is not available to a reproduction-dedicated optical information recording medium. The reproduction-dedicated optical information recording medium stores information which is not rewritable due to a protrusion/depression pattern formed on a substrate.
In the super-resolution reproducing method disclosed in Patent Document 2, a reproducing layer is formed on a record holding layer. In a reproduction of a signal, a signal recorded on the record holding layer is transferred to the reproducing layer only in an area whose temperature exceeds a certain value, that is, an area which is inside a spot of a light beam for a reproduction. This allows for a reproduction of a signal having a recording mark length shorter than an optical resolution limit. This improves (i) a linear recording density along a scanning direction with respect to a recording track and (ii) a track density which is determined by a gap (a track pitch) between tracks adjacent to each other and which is in a direction perpendicular to the scanning direction.
Although the conventional technique disclosed in Patent Document 1 allows for the reproduction of the signal having the recording pit length shorter than the optical resolution limit and therefore improves the linear recording density, this technique has a limit on the improvement in the linear recording density.
If the technique disclosed in Patent Document 1 improved the track density, the technique could provide a higher track density. Actually, however, it is impossible for the technique disclosed in Patent Document 1 to improve the track density because this technique does not improve signal leakage to a target recording track from a recording track adjacent to the target recording track, that is, crosstalk. The track density is determined mainly by the crosstalk.
Further, the layer which contributes to the super-resolution reproduction has some tens nm, that is, the layer is thin. Due to this, it is impossible for the layer to have sufficient reproduction durability. However, the thinner the layer contributing to the super-resolution reproduction is, the greater the effect obtained by the high-resolution reproduction becomes. That is, in the case where the layer contributing to the super-resolution reproduction is made thinner for the purpose of obtaining a greater effect of the high-resolution reproduction, such a problem occurs that the reproduction durability of the layer is reduced.
Further, the conventional technique disclosed in Patent Document 2 is the super-resolution reproducing technique for a magneto-optical recording/reproducing method. That is, although this technique improves both of the linear recording density and the track density, this technique is available only to the magneto-optical recording/reproducing method.
Therefore, it is impossible for the conventional technique of Patent Document 2 to carry out a reproduction of other methods, such as the reproduction-dedicated optical information recording medium, on which recording information cannot be rewritten due to the protrusion/depression pattern formed on the substrate. The reproduction-dedicated optical information recording medium is considered to be important to make a drive widely used.
In the technique disclosed in Patent Document 2, a reproducing film itself directly absorbs light and thereby generates heat so as to allow for the super-resolution reproduction. With this, material of the reproducing layer is likely to be subjected to a greater load. This causes such a problem that sufficient reproduction durability is not attained by the reproducing film.
Furthermore, a phase-change film, a dye film, or the like may also be used as a conventional super-resolution film. However, such a super-resolution film directly absorbs light or heat, so that the super-resolution film changes its composition or its phase. With this, material of the super-resolution film is likely to be subjected to a greater load. This causes such a problem that sufficient reproduction durability is not attained by the conventional super-resolution film.
Moreover, the phase-change film and the dye film are more expensive than film material generally used for an optical information recording medium. This causes such a problem that a super-resolution medium is more expensive than a conventional optical information recording medium.
As described above, the conventional super-resolution reproducing techniques have a lot of problems.    [Patent Document 1]
Japanese Unexamined Patent Application Publication, Tokukai, No. 2001-250274 (published on Sep. 14, 2001)    [Patent Document 2]
Japanese Unexamined Patent Application Publication, Tokukaihei, No. 3-93058 (published on Apr. 18, 1991)
DISCLOSURE OF INVENTION
The present invention was made in view of the foregoing problems, and an object of the present invention is to provide: (i) an optical information recording medium (a) which is capable of reproducing high-density recording information by improving a linear recording density and a track density and (b) which is excellent in reproduction durability; (ii) a recording/reproduction device of the optical information recording medium; and (iii) a reproducing method of the optical information recording medium.
In order to solve the foregoing problems, an optical information recording medium according to the present invention is arranged so as to be an optical information recording medium in which information has been recorded and from which the information is reproduced in response to reproducing light irradiated onto the optical information recording medium, the optical information recording medium, including: a light-transmitting layer or a transparent substrate; an information recording layer; and a substrate, the light-transmitting layer or the transparent substrate, the information recording layer, and the substrate being stacked in this order from a side from which the reproducing light enters, the information recording layer including at least a reproducing film whose complex refractive index at a light source wavelength λ of the reproducing light is changed by heat, and the following relationship being satisfied:0.67×(λ/NA)>TP>0.04×(λ/NA),where TP is a track pitch of a recording track formed on the substrate and NA is a numerical aperture of an objective lens for converging the reproducing light onto the optical information recording medium.
With this, when the reproducing light enters the reproducing film whose complex refractive index at the light source wavelength λ of the reproducing light is changed by the heat, the reproducing film itself or a layer (e.g., a light-absorbing film) which is adjacent to the reproducing film absorbs a part of the reproducing light and converts the part of the reproducing light into heat.
The reproducing film is heated by the heat thus converted, so that temperature distribution is provided to the inside of a light spot on the reproducing film. It is considered that a recording mark having a mark length equal to or shorter than an optical resolution limit is reproduced as follows: (i) In accordance with the temperature distribution, optical constants n and k of the reproducing film are changed. (ii) This increases or decreases a reflection rate inside the high-temperature area. (iii) Thereby, the spot of the reproducing light is reduced in a pseudo manner.
Thus, with the information recording layer including the reproducing film whose complex refractive index at the light source wavelength λ of the reproducing light is changed by the heat, it is possible to increase a linear recording density. Further, it is also possible to improve crosstalk in a direction perpendicular to a scanning direction, because the spot of the reproducing light is reduced in the pseudo manner as described above.
Furthermore, according to an experiment, it has been confirmed that reproduction durability is improved in the optical information recording medium including the reproducing film whose complex refractive index at the light source wavelength λ of the reproducing light is changed by the heat. That is, with the reproducing film whose complex refractive index at the light source wavelength λ of the reproducing light is changed by the heat, it is possible to provide an optical information recording medium which is excellent in reproduction durability.
Moreover, with the information recording layer including the reproducing film, it is possible to improve the crosstalk. This allows a track pitch to be reduced.
Specifically, the present invention allows the track pitch to be reduced to less than 0.67×(λ/NA), which is a limit of a track pitch of a conventional technique, thereby allowing a gap between tracks to become narrower than that of the conventional technique. Therefore, the present invention attains an increased track density, which is not achieved by the super-resolution media according to Patent Document 1 and the like. Thus, the present invention provides an optical information recording medium having a higher density.
That is, the present invention increases the track density, as well as the linear recording density. This makes it possible for the present invention to attain a high-density recording capacity, which is not achieved by an optical information recording medium using the conventional super-resolution technique.
There is a case where (i) an improved amount of crosstalk occurring from a recording track inwardly adjacent to a target recording track on the optical information recording medium differs from (ii) an improved amount of crosstalk occurring from a recording track outwardly adjacent to the target recording track on the optical information recording medium. This difference is caused by, for example, the following reason. Because the optical information recording medium is rotating during a reproduction, the center of the spot of the reproducing light which spot is reduced in the pseudo manner is not coincident with the center of the reproducing light. In order to deal with this, by setting the track pitch so that the relationship “TP>0.04×(λ/NA)” is satisfied as described above, it is possible to equalize (i) the improved amount of the crosstalk occurring from the recording track inwardly adjacent to the target recording track with (ii) the improved amount of the crosstalk occurring from the recording track outwardly adjacent to the target recording track. This further improves the crosstalk and increases the track density.
Further, an optical information recording medium according to the present invention is arranged so as to be an optical information recording medium in which information has been recorded and from which the information is reproduced in response to reproducing light irradiated onto the optical information recording medium, the optical information recording medium, including: a light-transmitting layer or a transparent substrate; an information recording layer; and a substrate, the light-transmitting layer or the transparent substrate, the information recording layer, and the substrate being stacked in this order from a side from which the reproducing light enters, and the information recording layer including at least a reproducing film mainly made of a metal oxide, and the following relationship being satisfied:0.67×(λ/NA)>TP>0.04×(λ/NA),where TP is a track pitch of a recording track formed on the substrate and NA is a numerical aperture of an objective lens for converging the reproducing light on the optical information recording medium.
With the information recording layer including at least the reproducing film mainly made of the metal oxide, it is possible to improve (i) the linear recording density and (ii) the crosstalk in the direction perpendicular to the scanning direction. This allows the gap between the tracks to be reduced. This makes it possible for the present invention to attain an increased track density, which is not achieved by the super-resolution media according to Patent Document 1 and the like. Thereby, the present invention provides an optical information recording medium having a higher density.
That is, it is possible for the present invention to increase the track density, as well as the linear recording density. This makes it possible for the present invention to attain a higher-density recording capacity, which is not achieved by the optical information recording medium using the conventional super-resolution technique.
Further, the optical information recording medium using the reproducing film mainly made of the metal oxide exhibits good reproduction durability. This is because that the metal oxide is chemically stable, and therefore is not easily damaged by the incidence of the reproducing light. This makes it possible for the present invention to provide an optical information recording medium whose reproduction durability is improved compared with the optical information recording media disclosed in Patent Document 1 and Patent Document 2.
Furthermore, it is possible to form the reproducing film mainly made of the metal oxide by means of a vapor deposition method or a sputtering method. That is, it is relatively easy to form the reproducing film mainly made of the metal oxide.
Moreover, it is possible to collect the metal oxide as metal by deoxidizing the metal oxide. This improves recyclability of an optical information recording medium.
In a reproducing device for reproducing the optical information recording medium according to the present invention, a reproduction is carried out such that a center of reproducing light is shifted from a center of a recording track by an amount which is more than 0.02×(λ/NA) but less than TP/4.
This makes it possible to equalize (i) a crosstalk amount with respect to a track inwardly adjacent to a target track on the optical information recording medium with (ii) a crosstalk amount with respect to a track outwardly adjacent to the target track on the optical information recording medium, during the reproduction of the optical information recording medium. This further improves track density, thereby making it possible to provide a reproducing device for an optical information recording medium having a high density.
In a reproducing method of reproducing the optical information recording medium according to the present invention, a reproduction is carried out such that a center of reproducing light is shifted from a center of a recording track by an amount which is more than 0.02×(λ/NA) but less than TP/4.
This makes it possible to equalize (i) a crosstalk amount with respect to a track inwardly adjacent to a target track on the optical information recording medium with (ii) a crosstalk amount with respect to a track outwardly adjacent to the target track on the optical information recording medium, during the reproduction of the optical information recording medium. This further improves track density, thereby making it possible to provide a reproducing method for an optical information recording medium having a high density.